1. Field of the Invention
The present invention relates to a digital copying machine, a facsimile or a laser printer that forms an image using electrophotographic technology, or a digital copying machine that combines these functions. More particularly, the present invention relates to an image forming apparatus that uses a multi-beam technique to form an image by scanning a plurality of lines with a plurality of beams, as well as a control method thereof.
2. Description of the Related Art
Conventionally, an image forming apparatus, which forms an electrostatic latent image on a photosensitive member by an electrophotographic process, using a laser scanning optical system that irradiates light such as laser beam light emitted from a light emitting device onto a drum-shaped electrophotographic photosensitive member as an image carrier, namely a photosensitive drum, is known.
In recent years, improvements in image forming speed and image forming density (resolution) are being sought with respect to this type of image forming apparatus. In response to these demands, an image forming apparatus has been realized in which an image clock for forming each picture element is speeded up in the main-scanning direction, and the rotating speed of a polygon motor is accelerated in the sub-scanning direction.
However, since there is a limit to the degree to which the rotating speed of the polygon motor can be accelerated, as another method of acceleration a multi-beam scanning optical system has been proposed that simultaneously and in parallel scans a plurality of laser beams on a photosensitive member at one scanning. Using this multi-beam scanning optical system, the rate of scanning by laser beams in forming an image on a photosensitive member is given by 1/(number of laser beams).
In a configuration that scans each laser beam on a photosensitive member using a multi-beam optical system, if variations occur in the production process of each optical element affecting their optical properties, the scanning magnifications in the main-scanning direction will not match and the image quality will decline. It is therefore necessary to perform processing to correct this inconsistency and restore the scanning magnifications in the main-scanning direction to be equal.
To solve this problem, it is necessary to allow higher quality images to be formed by making it possible to adjust the laser modulation rate as one parameter that determines scanning magnification in the main-scanning direction separately for each laser, and to scan each beam on the photosensitive member at a constant and equal scanning magnification. Thus, a method (see JPA 2001-013430) has been proposed which corrects the main-scanning magnification by disposing light detecting means (BD sensor: beam detect sensor) at the start point and end point in the main-scanning direction to detect the main-scanning magnification of each beam using the BD sensors and finely adjusting the image clock frequency of each beam.
The problem, in which image deterioration is caused by differences of scanning magnifications in the main-scanning direction due to differences in the scanning incident angle of laser beams in an image forming apparatus using the conventional multi-beam scanning optical system, will now be described with reference to FIGS. 1 to 4. FIG. 1 is a view illustrating a known laser scanning unit. The scanning direction of laser beams (an example of 4 beams is used in the figure) irradiated from unshown laser diodes is determined by a polygon mirror 102 that is rotationally driven at a predetermined rotational frequency by a polygon motor 103 (laser beams 104). These laser beams are controlled so as to scan the surface of a photosensitive drum 101 via a reflection mirror 105.
FIG. 2 is a view showing a part of FIG. 1 as viewed from above. By rotational driving of the polygon mirror in the direction indicated by the arrow in the figure the laser beams are scanned as shown in the figure, and BD sensors 106 and 107 are disposed in the optical path of the laser beams. The BD sensor 106 is disposed at the start point in the laser scanning direction and the BD sensor 107 is disposed at the end point in the laser scanning direction, and they perform detection of main-scanning magnification (detection of difference in scanning length) and output of a synchronizing signal for the main-scanning direction.
In this case, as shown in FIG. 1, if the scanning incident angle θ of the laser beams onto the photosensitive drum 101 is roughly the same for each beam, a decrease in image quality caused by a difference in the scanning incident angles of the laser beam does not occur.
However, as shown in FIG. 3, in an ordinary image forming apparatus, in order to reduce the light returned by reflection from the photosensitive drum, or due to constraints on the image forming apparatus such as miniaturization, the scanning incident angle of the laser beams differs as shown by θ1 and θ2 in the figure. Therefore, although the beams had the correct scanning magnification at the disposition location of the BD sensor, the scanning magnification differs by the difference in the scanning incident angles of the laser beams on the photosensitive drum as shown in FIG. 3. As a result, the scanning lengths of the laser beams on the photosensitive drum 101 differ as shown in FIG. 4.
FIG. 4 is a view illustrating toner images that are formed on the photosensitive member by the four laser scanning beams LD1 to LD 4 shown in FIG. 3. As shown in FIG. 4, these laser scanning beams LD1 to LD 4 produce the differences in the scanning lengths, more specifically, in the lengths of the toner images formed on the photosensitive drum in the scanning direction. When the scanning lengths of the laser beams on the photosensitive drum 101 differ in this manner, vertical line fluctuations and the like occur and thus a problem arises in that image quality may decrease.